In the background art, there has been known a work vehicle provided with a travel vibration suppression device which is called a ride control device. The ride control device is a device in which a hydraulic pressure accumulator is connected through a control valve to a lift cylinder hydraulic circuit which supplies hydraulic oil to a lift cylinder for driving a work tool. In the ride control device, the hydraulic oil is allowed to circulate between the lift cylinder and the hydraulic pressure accumulator when the control valve is opened. The fluctuation of bottom pressure generated in the lift cylinder due to the vertical motion of the work vehicle which is travelling can be absorbed to the hydraulic pressure accumulator so as to reduce the impact acting on the vehicle body. The control valve is fundamentally changed over by manual operation performed on a ride control switch by an operator. In the background art, in order to automatically change over the control valve in accordance with the operation situation of the work vehicle, there has been also known a technique in which the control valve is changed over from a closed state to an open state automatically when the vehicle speed reaches a predetermined set speed or higher (for example, see Patent Literature 1).
However, in the configuration in which the control valve is opened and closed automatically in accordance with only the vehicle speed, the control valve may be opened and closed in a state which is not intended by the operator. As a result, a sense of discomfort or a sense of insecurity in operation may be easily given to the operator. For example, description will be made by way of example in the case where in which a bucket is provided as a work tool for performing excavation work, hauling work and loading work onto a dump truck or the like. When the vehicle speed is higher than a set speed, the control valve is automatically changed over from the closed state to the open state in spite of the excavation work. Thus, the force acting on the bucket escapes to the hydraulic pressure accumulator through the lift cylinder due to the damper effect of the hydraulic pressure accumulator, so as to give the operator a sense of discomfort as if the start timing of excavation has been delayed. In addition, when the vehicle speed is higher than the set speed during the loading work, the control valve is automatically changed over from the closed state to the open state. As a result, the oscillation of the bucket increases due to the damper effect of the hydraulic pressure accumulator, so as to give a needless sense of insecurity to the operator. In order to solve such problems, the applicant of the present application has already made a proposal in which a position of excavation, a position of hauling and a position of loading are set in a controller in advance, and a control valve is kept in a closed state in spite of a vehicle speed not lower than a set speed when a bucket is in a position not higher than the set position of excavation or not lower than the set position of loading (Japanese Patent Application No. 2011-56644). In this manner, the bucket can be prevented from oscillating during excavation work or during loading work, so that the sense of discomfort or the sense of insecurity can be removed from the operator.
In addition, in the background art, a work vehicle such as a wheel loader is also mounted with a transmission control device which automatically changes a speed stage of a transmission when the vehicle speed reaches a set speed. However, in the configuration in which the speed stage of the transmission is automatically changed in accordance with only the vehicle speed, shift-up operation may be performed against the operator's intention to accelerate the work vehicle when the vehicle speed is higher than the set speed. Thus, the work efficiency may deteriorate instead. For example, the following work is performed when soil etc. fully loaded on the bucket is loaded onto a dump truck. That is, the work vehicle is moved forward from the position of excavation toward the stop position of the dump truck. As soon as the work vehicle comes close up to a predetermined position from the dump truck, the amount of accelerator pedal depression is reduced to lower the vehicle speed. The bucket is lifted up to the loading height onto the dump truck while the work vehicle is moved forward due to an inertia force. The work vehicle is stopped in the loading position of the soil etc. However, when the vehicle speed is high, shift-up operation is carried out in spite of a small amount of accelerator pedal depression. Thus, the work vehicle may be accelerated. On this occasion, the operator must brake and stop the work vehicle in the loading position of the soil etc. Thus, the work efficiency deteriorates. In order to solve such a problem, the applicant of the present application has already proposed a transmission control device in which a loading position is set in a controller in advance, and shift-up operation by a transmission device is inhibited when the vehicle speed reaches a set speed or higher as long as a bucket is not lower than the set loading position (for example, see Patent Literature 2). In this manner, needless braking operation can be avoided so that the work efficiency of the work vehicle can be improved.